Add styled alerts to markdown

[Hiviexd]

Resolves #13064 (with applied feedback from #osu-wiki)

Enabled for wiki and news.

Also supports an inline variant per wiki maintainer request.

Preview

Initial implementation commentary

initial implementation used the GitHub custom syntax detailed in the linked issue via a custom parser. after a conversation with clayton and Wala in #osu-wiki (link), we decided it would be best to rework this using existing syntax of CustomContainerInline (similar to flags), and let CSS selectors do the heavy lifting, which eliminates the need of custom parsers altogether.

finalized syntax is the following:

> ::{alert=caution}:: **Caution**
> This is *critical* content demanding of the user's attention.

this results in an alert.

> ::{alert=note}:: **See also:** [Ranking criteria](/wiki/Ranking_criteria) and [Beatmap ranking procedure](/wiki/Beatmap_ranking_procedure)

this results in an inline alert.

---

Available styleblock attributes are:

• ::: alert-note
• ::: alert-tip
• ::: alert-notice
• ::: alert-warning
• ::: alert-caution

Syntax is the following:

::: alert-note
**See also:** [Ranking criteria](/wiki/Ranking_criteria)
:::

This results in an inline/compact alert.

::: alert-caution
**Caution**
This is *critical* content demanding of the user's attention.
:::

This results in a regular alert.

[notbakaneko]

You can whitelist extra attributes in attributes_allowed and add attributes to blockquotes:

{data-alert=note}
> **line1**
> line2

custom inline containers are for customizing the rendering of inline elements, not abusing to make a block level element behave differently.

There's also the existing style block syntax for infoboxes that can be extended to support more customization:

::: Someotherclass
alert
:::

[notbakaneko]

There's also the argument that blockquote shouldn't be used for things that aren't quotes, this is more like a div with a left border

[Hiviexd]

went all in with style blocks, see PR description for the syntax I chose.